Good size for banner image?

[Senkusha]

I'm wanting to make a new website banner image for my forum.  In your experience, what's an ideal size for web banners?

[Kindred]

That depends, completely, on your site layout (theme)....

[Antechinus]

My 2c: you can go as wide as you like, but don't go too tall. I find it annoying when half the screen height is filled with a huge banner, so I assume some other people find it annoying too. A couple of hundred px tall is fine on desktop. Works pretty well even on mobile too, although you may have to think about how you want things to scale on mobile.

[Diego Andrés]

Anything above 250px starts to get annoying because some of your users will browse on tablets or phones.
And if you don't really care about the details on your banner, you don't have to worry about the positioning too much. A classic example of this would be text or characters you want people to see.

[Steve]

You mean like this site: https://friday-night-lights2.proboards.com/